The invention relates to an optical waveguide fiber and optical components. More particularly, the invention relates to a dispersion compensating module and a mode converter, coupler and dispersion compensating optical waveguide fiber useable therewith.
Dispersion compensating fibers used in telecommunications systems 10, such as illustrated in FIG. 1, correct for the unwanted effects of dispersion of the transmission fiber 12. Transmission fibers 12 preferably have a large effective area (e.g.,  greater than 60 xcexcm2, and more preferably greater than 70) and propagate light signals in the fundamental mode (LP01). An example of a transmission fiber is LEAF(copyright) optical fiber manufactured by Corning Incorporated of Corning, N.Y., which is designed to operate primarily at about the 1550 nm operating window. In some systems, compensation takes place within a module 11 having a length of Dispersion Compensating (DC) fiber housed within it. A section 13 of transmission fiber terminates at the module 11 and is coupled with the DC fiber. After being dispersion compensated, the DC fiber is again coupled with the transmission fiber 12 and the signal continues along a continuing portion 14 of the transmission system 10. FIG. 1 illustrates a simple system deployment. It should be understood that typical transmissions systems include other devices such as amplifiers before and after the module, add/drop devices, etc.
One solution described in U.S. Pat. No. 5,185,827 and shown in FIG. 2, compensates for the dispersion of the transmission fiber by providing a dispersive waveguide element which transmits the light signal at a higher-order LP11 mode. An optical mode converter is utilized to convert the incoming signal from the fundamental mode carried by the transmission fiber to the higher-order mode LP11 mode that is supported and carried by the dispersive waveguide element. Similarly, once the dispersion compensation is achieved, a second optical mode converter converts the light signal back to the fundamental mode (LP01). However, transmission in the LP11 mode has a problem that the signal may be split into multiple modes due to slight imperfections in the fiber""s circular geometry. This has the effect of undesirably distorting the transmitted signal.
Thus, it should be recognized that the properties of the DC fiber used in the dispersion compensating module are vitally important to the performance of the overall optical transmission system.
According to a first embodiment of the invention, an optical waveguide fiber suitable for use as a dispersion compensating fiber is provided with improved properties such that it may advantageously support light propagation in an LP02 mode. Preferably, propagation is supported at a wavelength of about 1550 nm and for a sufficient distance to compensate for dispersion of another fiber, for example an optical transmission fiber.
According to another embodiment of the invention, a Dispersion Compensating (DC) waveguide fiber is provided comprising a plurality of core segments. The refractive index profile of the DC fiber is selected to exhibit properties such that an LP02 mode is supported and propagated thereby at a wavelength of about 1550 nm. Upon conversion to the LP02 mode, preferably by an all-fiber mode converter according to another embodiment of the invention, the incoming signal is propagated within the DC fiber for an appropriate length (generally about 0.5-3.0 km, depending on the transmission fiber used). The DC fiber is designed to compensate LP02 mode for the dispersion effects of the transmission optical waveguide (the primary fiber transmitting in an LP01 mode).
Preferably, the transmission waveguide, for which dispersion correction is occurring, has a length greater than 25 km, and more typically on the order of between about 50 km-100 km. The invention described herein advantageously allows for a very short segment of DC fiber to accomplish the dispersion compensation. For example, in one embodiment, less than 1/100th of the length of the transmission fiber may be required for compensation of certain transmission fibers, for example Corning""s LEAF(copyright) optical fiber.
In accordance with another aspect of the invention, the DC optical waveguide fiber exhibits a kappa value between about 10 nm and about 500 nm; where kappa is the ratio of dispersion in the LP02 mode at about 1550 nm divided by the dispersion slope in the LP02 mode at about 1550 nm. In accordance with a more preferred embodiment, the kappa value is in the range between about 30 nm and about 70 nm. According to another embodiment, the DC waveguide preferably has an effective area greater than about 30 xcexcm2 at about 1550 nm, more preferably greater than about 60 xcexcm2, and more preferably yet between about 30 xcexcm and 150 xcexcm2, and most preferably between about 50 xcexcm2 and about 90 xcexcm2.
In a preferred embodiment of the invention, the fiber comprises a plurality of, preferably at least three core segments. Preferably, first and third segments of the plurality of segments comprise a dopant such as germanium to raise the index of refraction of the core a sufficient amount with respect to the cladding to achieve the desired xcex94%. Alternatively, any other suitable dopants such as phosphorous may be employed. Moreover, fluorine doping may be employed to lower the refractive index of a second core region and/or the clad region as compared to the core.
The geometry of the refractive index profile of the DC fiber is selected accordingly to enable transmission of the LP02 mode over substantial distances (e.g.,  greater than 0.5 km). For example, the structure, i.e., the radius of the various segments, their width dimensions, and their xcex94% values are selected in accordance with the invention as described in the several examples provided herein.
In accordance with one preferred embodiment, the waveguide comprises a structure with:
(a) a first core segment having an outer radius in the range between about 3 xcexcm and 8 xcexcm and a xcex94% peak in the range between about 1.0% and 2.5%,
(b) a second core segment having an outer radius in the range between about 7 xcexcm and 13 xcexcm and a xcex94% peak in the range between about 0.3% and xe2x88x920.5%, and
(c) a third core segment having an outer radius between about 10 xcexcm and 20 xcexcm and a xcex94% peak in the range between about 0.2% and 1.0%.
Other embodiments and more preferred values of radii, xcex94% or combinations thereof are described more fully in the specification and appended claims. Fibers with these ranges of radii and xcex94% enable transmission in the LP02 mode.
In accordance with another preferred embodiment, the waveguide fiber comprises:
(a) an effective area in the range between about 50 xcexcm2 and 90 xcexcm2 at about 1550 nm and in the LP02 mode,
(b) a dispersion value at about 1550 nm and in the LP02 mode between about xe2x88x9250 and xe2x88x92400 ps/nm/km, and
(c) a dispersion slope value at about 1550 nm and in the LP02 mode between about xe2x88x920.01 and xe2x88x9220 ps/nm2/km.
Other preferred values of effective area, dispersion, dispersion slope, kappa or combinations thereof are more fully described in the specification and appended claims.
According to another embodiment of the invention, a dispersion compensating optical waveguide includes a plurality of core segments, the refractive index profile of which is selected to exhibit an effective area between about 30 xcexcm2 and 150 m2 wherein the dispersion compensating optical waveguide is capable of propagating light in the LP02 mode a sufficient distance at about 1550 nm, upon being converted from an LP01 mode, to be capable of compensating for dispersion of a length of fiber transmitting in the LP01 mode. Preferably, the fiber transmitting in the LP01 mode is a long-haul waveguide having a length greater than about 25 km. More preferably, the transmission fiber may be a fiber, such as LEAF(copyright) optical fiber available from Corning Incorporated, that exhibits an effective area greater than about 65 xcexcm2 in the LP01 mode. Preferably, the DC optical waveguide has a length between about 0.5 km and about 3 km, thus providing a segment that is short enough to conveniently package within a compact dispersion compensating module.
In accordance with another embodiment of the invention, a dispersion compensating module is provided including a reflective fiber grating to convert light propagating in a first mode into light propagating in a second mode. Most preferably, the module comprises a coupler adapted to couple a first fiber that is adapted to propagate light in a first mode with a second fiber. In accordance with this aspect of the invention, a reflective fiber grating is operatively connected to the coupler; the fiber grating being adapted to convert light propagating in the first mode into a second mode. In the compensating module in accordance with another aspect thereof, the second fiber is operationally and optically coupleable through the coupler to the reflective fiber grating and the second fiber may propagate light in a second mode. According to a preferred embodiment of the invention, the first fiber is a transmission fiber and the second fiber is a dispersion compensating fiber. Preferably, the first mode is an LP01 mode and the second mode is an LP02 mode.
In accordance with a preferred embodiment, the dispersion compensating module comprises a mode converter and a dispersion compensating fiber. The mode converter is operatively coupleable with a transmission waveguide; the transmission waveguide being adapted to propagate light in a first mode. Within the mode converter is a reflective fiber grating capable of converting the first mode into a second mode. A dispersion compensating fiber is operatively coupled to the mode converter and the dispersion compensating fiber is adapted to propagate light in the second mode to compensate for dispersion of the transmission fiber.
According to another embodiment of the invention, the dispersion compensating module comprises a mode converter adapted for operatively coupling with an optical transmission waveguide, the transmission waveguide propagating light in a first mode. The mode converter includes a reflective fiber grating that is adapted to convert the first mode into a second mode. The module also includes a dispersion compensating fiber, operatively coupled to the mode converter, adapted to propagate light in the second mode. The module preferably also includes a coupler adapted to couple light propagating in the first mode into the reflective fiber grating and which is further adapted to couple light propagating in the second mode into the dispersion compensating fiber.
In accordance with another embodiment of the invention, an optical mode converter is provided comprising an optical fiber coupler adapted to operatively couple light propagating in a first mode in a first fiber into a second fiber, and a reflective fiber grating operatively coupled to the second fiber, the grating being capable of converting light propagating in a first mode into a second mode wherein the second fiber extends from the optical fiber coupler and is adapted to propagate light in the second mode. Preferably, the first fiber is a fiber pigtail adapted to operatively couple to an optical transmission waveguide propagating light in an LP01 mode. Most preferably, the reflective fiber grating converts the LP01 mode into an LP02 mode; the fiber grating being operatively coupled with the pigtail through, for example, an optical fiber coupler.
In one embodiment, a fiber interconnect operatively couples the reflective fiber grating with a DC fiber; the DC fiber adapted to propagate light in the LP02 mode. The reflective fiber grating preferably includes a plurality of longitudinally spaced portions that have been exposed to UV radiation to vary those respective portions"" refractive index. Preferably, the longitudinal spacing of the portions are spaced at intervals that vary by up to 3% from a beginning to an end of the reflective fiber grating. It should be recognized that a broader spacing variation may be utilized if a broader grating bandwidth is desired. Various characteristics of the preferred conversion fiber upon which the fiber grating is written are described herein. In one embodiment, the conversion fiber comprises boron, germanium and phosphorous doped silica.
According to another embodiment of the invention, an optical fiber coupler is provided wherein the propagation constants (in a particular mode) of a first and second fiber therein are matched by stretching a portion of one of the fibers prior to fusion thereof. In more detail, the coupler comprising a first optical fiber within the coupler having a first propagation constant in a first mode, and a second fiber within the coupler, the second fiber having a second propagation constant in an undeformed portion thereof and in the first mode that is different than the first propagation constant, the second fiber including a necked-down portion formed on a glass portion thereof which is formed prior to fusion of the fibers, the necked-down portion having a dimension such that a third propagation constant in the necked-down portion substantially matches the first propagation constant wherein coupling of light between the fibers in the first mode is enhanced. Further details of the dispersion compensating module and the mode converter, coupler and various fibers included therein are in the attached disclosure, claims and drawings to follow.
The following definitions are in accord with common usage in the art.
The refractive index profile is a plot of the relationship between refractive index and waveguide fiber radius. It is generally provided as a xcex94% as defined below.
A segmented core is one that has at least a first and a second waveguide core segment positioned at a radial distance from the waveguide centerline. Each segment has a respective refractive index profile.
The radii of the segments of the core are defined in terms of the beginning and end points of the segments of the refractive index profile. FIG. 5, for example, illustrates the definitions of radii R1, R2 and R3 used herein. The radius R1 of the first index segment 18, is the length that extends from the waveguide centerline to the point at which the profile, when extrapolated with a tangential line, intersects the innermost portion of a tangentially extrapolated portion of the next adjacent segment. The outer radius R2 of second segment 19 extends from the centerline to an outermost radial point of the second segment at which the tangentially extrapolated edge portion of the inner radius of the third core segment intersects the outermost point of the second segment. The outer radius R3 of third segment 20 extends from the centerline to the radius point at which the descending tangential portion of the third core segment intersects the zero xcex94%, if for example, there are additional segments utilized. The width of each segment 18, 19, and 20 respectively is measured with respect to the radii R1, R2-R1, and R3-R2, respectively.
The effective area is defined herein as:
Aeff=2xcfx80(∫E2 r dr)2/(∫E4 r dr),
where the integration limits are 0 to ∞, and E is the electric field associated with the mode in which the light is propagated and r is the radius within the integrated interval.
The term xcex94% represents a relative measure of refractive index defined by the equation:
xcex94%=100xc3x97(ni2xe2x88x92nc2)/(2ni2)
where ni is the refractive index in any region i along the profile, and nc is the refractive index of the cladding region, unless otherwise specified.
It is an advantage of the present invention that the DC waveguide fiber has greater effective area than prior DC fibers, thus providing lower nonlinear effects. This higher effective area is achieved by light transmission in the LP02 mode. This has the advantageous effect of reducing nonlinearities in the signal transmission.
It is another advantage of the present invention that the DC waveguide fiber propagates light signals in the higher order LP02 mode enabling high negative dispersion and negative slopes and thereby allowing compensation with shorter lengths of DC fiber. For example, in a preferred embodiment for use with LEAF(copyright) optical fiber, the length of DC fiber required may be less than {fraction (1/100)}th of the transmission fiber""s length. This enables shorter DC fiber lengths and thus lower losses as well as smaller DC modules. In particular, because the LP02 transmission mode exhibits circular symmetry (an even symmetry mode), it is desirably very tolerant of circularity variations in the fiber. The present invention dispersion compensating fiber enables their use in such devices over a wide range of wavelengths (larger bandwidth) and with low attenuation.
Therefore, the present invention solves the problem of mode splitting when transmission is propagated in the prior art LP11 mode.
An advantage of another embodiment of the invention is that the mode conversion and dispersion compensation is accomplished with an all fiber based approach, thus enabling compact, robust and cost effective mode conversion and dispersion compensation.
Other aspects and advantages of the invention will be understood with reference to the following detailed description, claims and appended drawings.